Detecting CDOM Fluorescence Using High Spectrally Resolved Satellite Data: A Model Study - Towards an Interdisciplinary Approach in Earth System Science - page 102

locally and globally to examine water quality, chemical composition of dissolved
organic matter, trace water masses and study dynamical processes involving CDOM.
CDOM

cant inelastic process in natural waters, along with
vibrational Raman scattering (VRS) and chlorophyll

fl fluorescence is a signi

uorescence.
Between processes of absorption and emission some energy is dissipated, so the
energy emitted is lower than the energy absorbed. The wavelengths at which a
photon is absorbed and emitted depend mainly on the molecule itself, but can also
depend on several environmental factors, e.g. pH, temperature, a presence of certain
metals (Hudson et al. 2007 ). Fluorescent CDOM is a complex mixture of many
fl

fl

fluorescent compounds, and individual fluorophores are hard to identify.

Because of the shift of wavelength in the

fl

fluorescence process, its effect on the
backscattered radiation at the top of atmosphere is possibly identi

fl

lling-
in of Fraunhofer lines, which are spectrally narrow, often saturated absorption
features in the solar spectrum. Other inelastic processes have been already retrieved
from hyperspectral satellite data with the Differential Optical Absorption Spec-
troscopy (DOAS) method (e.g. Vountas et al. 1998 , 2003 , 2007 ). In this work we
test the use of the

ed in the

uorescence to char-
acterize CDOM composition and concentration independent from chlorophyll. This
method would also help to improve chlorophyll retrievals in regions of high CDOM
concentrations.

filling-in signal originating from CDOM fl

2 Methods

2.1 DOAS Retrieval Technique

Different variants of the DOAS technique have been applied for several years for
the retrieval of atmospheric trace gases from satellite measurements (e.g. Burrows
et al. 1999 ; Richter et al. 2005 ), and have lately been extended to the aquatic
domain as PhytoDOAS (Vountas et al. 2007 : Bracher et al. 2009 ; Sadeghi et al.
2012 ). The DOAS method is based on Beer-Lambert

is law (e.g. Rozanov and
Rozanov 2010 ) and determines the amount of molecular absorbers along the
effective optical light path by

'

fitting and scaling spectra within a given wavelength
window. The broad-band effects (e.g. Rayleigh and Mie scattering) are removed by
a low-degree polynomial that is also

fitting in the DOAS method is
formalized as a least-squares minimization and is described by the following
equation:

tted. The

2

X

N
n¼1 r n ðkÞ S n r R ðkÞ S R r V ðkÞ S V r f ðkÞ S f

X

M

m

sðkÞ a m k ! min : ð 1 Þ

m¼0

ln I ðkÞ
I 0 ðkÞ

are
the measured backscattered radiance and extraterrestrial irradiance, respectively;

Here, sðkÞ ¼ is the so-called slant optical density, I ðkÞ and I 0 ðkÞ